1. Field of the Invention
The present invention relates to a flow quantity control valve used for such as an exhaust gas recirculation valve device controlling the flow quantity of the high-temperature exhaust gas exhausted by the internal combustion engine of an automobile, for instance.
2. Description of the Related Art
Conventionally, a variety of exhaust gas recirculation valve devices (hereinafter referred to as EGR devices) have been known. FIG. 6 is a cross sectional view to show an internal structure in the vicinity of a flow quantity control valve portion used for a conventional EGR device. In the drawing, the reference numeral 1 is a housing composed of cast iron. Inside the housing 1, there are provided a first fluid passage 2 and a second fluid passage 3 through which the high-temperature exhaust gas passes. There is provided a valve seat 4 between the first fluid passage 2 and the second fluid passage 3. Within the housing 1, there is provided a roughly cylindrical stay portion 5 closely adjacent to the second fluid passage 3. At a center of the bottom of stay portion 5, there is formed a through hole 6 which connects an inside of the stay portion 5 to an inside of the second fluid passage 3. Within the through hole 6, a valve stem 8 is provided slidably along the directions shown by the arrows A and B through the intermediary of a bearing 7. On one end of the valve stem 8, there is fixed a valve disk 9 that can engage and disengage with the valve seat 4, and on a concave portion 8a formed in a vicinity of the other end of valve stem, there is a fixed spring holder 11 through the intermediary of a bush 10. Between the spring holder 11 and an internal bottom part of the stay portion 5 of the housing 1, there is provided a valve spring 12 which energizes the valve stem 8 in the direction shown by the arrow B to engage the valve disk 9 with the valve seat 4.
On an upper edge part of the stay portion 5 of housing 1, there is fixed a driving means 13 by means of screws 14 which has a driving shaft 13a to advance the valve stem 8 in a direction shown by the arrow A against an energizing force of the valve spring 12, to thereby disengage the valve disk 9 from the valve seat 4. As for the driving means 13, for example, a DC motor, a stepping motor, a linear solenoid and the like are usually used.
In such an EGR device, when the driving shaft 13a retreats backmost position in the direction shown by the arrow B and the valve disk 9 which is fixed on the valve stem 8 engages with the valve seat 4, it is necessary that a bottom end of the driving shaft 13a of driving means 13 has properly abutted with a top end of the valve stem 8 neither too much nor too little. However in actual, when mounting the driving means 13 on the housing 1, there may be a possibility that the valve disk 9 does not abut with the valve seat 4 because of a dimensional error of the components even when the driving shaft 13a retreats backmost position. For this reason, a spacer 15 having a suitable thickness is placed between the upper edge part of the stay portion 5 of the housing 1 and the bottom end of the driving means 13, and the distance between the abutting surfaces is thereby adjusted.
The operation will next be described as below.
In the valve closing state because the driving shaft 13a of the driving means 13 retreats in the direction shown by the arrow B and the driving shaft does not thrust the valve stem 8 in the direction shown by the arrow A, the valve disk 9 is energized in the direction shown with the arrow B by the energizing force of the valve spring 12 and is engaged with the valve seat 4 (in a valve closing state). Therefore, the first fluid passage 2 is disconnected from the second fluid passage 3. Subsequently, when the valve is opened, the driving means 13 is driven to advance the driving shaft 13a in the direction shown by the arrow A and thereby the valve stem 8 is advanced to disengage the valve disk 9 from the valve seat 4 (in a valve opening state). In this valve opening state because the first fluid passage 2 is connected to the second fluid passage 3, the high-temperature exhaust gas flows from the first fluid passage 2 to the second fluid passage 3 and thereby the housing 1 constituting the second fluid passage 3 is heated up to about 350° C.
However, because the flow quantity control valve used for the conventional EGR device has an structure as described above, the valve spring 12 that is in contact directly with the housing 1 heated by the exhaust gas, is also exposed to the high temperature. For this reason, damages such as the breakage of the valve spring 12 and a reduction of the energizing force are caused. Because of the reduction of the energizing force, the valve disk 9 and the valve seat 4 disorderedly repeat the engagement and disengagement therebetween, to thereby cause abnormal sounds and the valve closing state cannot be properly maintained. As a result, there is a problem that the gas flow cannot be precisely controlled.
Moreover, in the flow quantity control valve used for the conventional EGR device, when a foreign object is caught within a coil of the valve spring 12, the caught foreign object changes the contraction and expansion stroke of the valve spring 12. As a result, there is also a problem that the valve closing state cannot be properly maintained.
By the way, Japanese Patent Laid Open Publication Hei 11-270417 discloses a flow quantity control valve having a similar structure to that of the flow quantity control valve used for the conventional EGR device shown in FIG. 6.